Determine reference_chirp_mass for MBGravitationalWaveTransient from prior when it is not specified
reference_chirp_mass
of MBGravitationalWaveTransient
should be less than the prior minimum of chirp mass to construct conservative frequency bands. This MR makes a small change on MBGravitationalWaveTransient
so that it automatically uses the prior minimum of chirp mass if reference_chirp_mass
is not specified. This also reorganizes tests for MBGravitationalWaveTransient
and adds tests for this change.